Rhododendrons are popular flowering shrubs that bloom in the Spring. They prefer a sheltered location with dappled sunlight and acidic soils to for flowering.
Rhododendrons do not flower if emerging flower buds have been damaged by a late frost or developing flower buds have been removed because of pruning at the wrong time of year. Drought, alkaline soils, lack of sun and nitrogen fertilizer can also prevent rhododendrons from flowering.
Keep reading to learn the cause of why your rhododendron is not flowering and how to solve it…
1. Pruning at the Wrong Time of Year (Removes Flower Buds)
Rhododendrons (as well as azaleas camellias and clematis) develop their flower buds around August which stay dormant over Winter to be displayed in the following Spring or Summer.
If your prune your rhododendron back with a hard prune in the Fall then you likely remove all the flower buds and the rhododendron won’t flower the following year.
Once you have pruned rhododendrons back (at the wrong time) they are unlikely to flower until 2 years time.
Rhododendrons do not require annual pruning in the same way that roses do and they flower on old wood rather then on new seasons growth.
However it can be necessary to prune back and improve the shape of rhododendrons as well as cut away old unproductive wood to keep the plant looking healthy.
Prune rhododendrons back straight after flowering otherwise you risk cutting off the developing flower buds for next year and you have to tolerate a year without flowering.
Pruning immediately after flowering gives the rhododendron enough time to grow back and set buds for next years flowering.
For all the best practices here is a YouTube video on how to prune rhododendron:
2. Not Enough Sunlight for Flowering
To ensure rhododendrons flower it is important to find the right balance of light and shade.
If the rhododendrons is in complete shade without even any bright indirect light then the plant does not have the resources or energy required to develop flower buds and display flowers.
Rhododendrons grow and flower in their native environment under a tree canopy which provides dappled light throughout the day.
However if the rhododendron is in full sun it can suffer leaf burn and drought stress.
Ideally you should try to replicate the rhododendron’s natural environment by planting rhododendron in an area of your garden with dappled light or partial shade.
Transplant your rhododendron (if possible) in the Spring or Fall to an area with more light to encourage flowering or cut back some overhanging tree limbs to allow more light to reach the rhododendron so it can develop flower buds and flower the following year.
Rhododendrons can tolerate morning sun (to promote flowering) followed by shade in the midday and afternoon when the sun as at its most intense.
3. Too Much Fertilizer, Fewer Flowers
If you apply a high concentration of fertilizer or apply too frequently, this can prevent flowering as the nitrogen in the fertilizer can promote foliage growth at the expense of flowers if applied incorrectly.
Rhododendrons are not as heavy feeders as other flowering shrubs such as roses and if the soil is nutrient rich or it has been prepared with organic matter (such as compost) before planting then an annual fertilizer application is often not necessary for promoting flowers.
However rhododendrons in poor, sandy soil or in pots can benefit from additional fertilizer to give them the require nutrients for healthy growth and the resources to flower.
For Rhododendrons azaleas and camellias I always use a specialised fertilizer from miracle-gro which has the right balance of nutrients at the right concentration to support flowers.
With a specific fertilizer you can easily avoid adding too much nitrogen with various fertilizer and take out the guess work to ensure the rhododendron can flower.
If you have lots of foliage and no flowers and you have used a lot of fertilizer, just leave the rhododendron to recover for a year without adding any fertilizer and if the plant is healthy it should flower the following year and form flower buds.
4. Frost Damage Flower Buds
Frost damage is often a problem with plants such as rhododendrons which form next years flower buds (usually from around August) after this years display of flowers.
It is usually a late frost in the Spring as the flowers begin to emerge that turns the flower buds a brown colour which of course prevents flowering.
The leaves of the rhododendron can often tolerate the frost but the flower buds are still vulnerable.
There is not much you can do to save flower buds once they have been damaged by frost and it can be a difficult problem to mitigate and whilst it is best practice to remove severely frost damaged flower buds, this won’t stimulate new flowers as rhododendrons are not repeat flowering.
However in climates that experinece freezing temperatures in Winter it is a good idea to plant (or transplant) your rhododendron in a relatively sheltered area of the garden such as under a tree in a corner with some protection.
Rhododendrons that are out in a open area are at higher risk of suffering frost damage to their flower buds.
You can also mitigate some damage with the use of horticultural fleece to protect flower buds before a late frost occurs, however I find it is more practical to find a sheltered planting area for your rhododendron.
5. Drought Stress Prevents Flowering
Rhododendrons requires well draining yet moist soil to grow and flower. If the soil is too dry then the plant is stressed and the flowers buds do not open in the Spring.
The soil around rhododendrons can be dry for several reasons:
- Rhododendrons grow well in the dappled light of a tree canopy, however if the canopy is too dense it can largely intercept and deflect rainfall away from your rhododendrons resulting in dry soil.
- Tree roots can also compete with rhododendrons for moisture in the soil which reduces the available water for your rhododendrons.
- Too much sun can also increase soil evaporation and water loss from the leaves.
- Drought or fast draining soil can cause the soil to be too dry for the rhododendron to display flowers as it requires consistently moist soil.
If your rhododendron has suffered significant drought stress then it may not flower this year however this can be easily addressed so that it flowers well the following year.
The solution is to add a layer of mulch around the rhododendron to help conserve water, add nutrients to the soil and improve the soils structure.
Apply a 1 inch thick layer of compost, leaf mould or well rotted manure to the surface of the soil around the rhododendron.
Organic materials such as compost, leaf mould or manure have an exceptional capacity for retaining water which helps to keep the roots cool and the rhododendron hydrated so that it can flower.
Ensure that the soil stays consistently moist throughout the growing season so the plant can flower by testing the soil to a fingers depth to detect moisture.
If the soil is beginning to dry out, give the rhododendron a generous soak to ensure it stays hydrated and to encourage the roots to establish in the soil.
With careful monitoring and watering when required and an application of mulch in the Spring the rhododendron should have the resources to display flowers.
6. Alkaline Soil Prevents Rhododendrons From Flowering
Rhododendrons require acidic soil to grow and flower properly. If the soil is neutral or even alkaline then this can stress the rhododendron causing it not to flower and can potentially kill the plant.
Rhododendrons thrive in acidic soil that is between pH 4.5-6.0
Whilst they can live in soils that are slightly less acidic (7 is pH neutral) they tend to flower less and grow poorly.
The easiest and often best way to find out your soils pH is to talk to a neighbour who is a keen gardener or horticulturalist in the area as they are likely to know the soil conditions.
Otherwise you can buy a soil gauge from amazon or a garden centre to test the pH of your soil to see if it is in the optimal range from rhododendrons to grow and flower.
In areas with alkaline soil it is better to grow your rhododendrons, azaleas and camellias (all prefer acidic soil) in pots or containers and use ericaceous (acidic) compost as amending alkaline soil to suit an acidic plant is difficult.
Key Takeaways:
- The reason for rhododendrons not flowering are because of pruning the wrong time of year, frosts damage to the developing flower buds, drought, alkaline soils too much nitrogen fertilizer or a lack of sunlight.
- Pruning during the Fall can remove the developing flower buds which display flowers of the following year.
- A late frost can damage flower buds (which turn brown) and prevent rhododendrons from displaying flowers.
- Too much nitrogen fertilizer can promote foliage growth at the expense of flowers for your rhododendron.
- Rhododendrons require consistently moist soil with an alkaline pH and dappled light to display flowers.